1. Field of the Invention
The present invention relates to a power supply circuit and, more particularly, to a power supply circuit of the voltage change-over type for use in an audio amplifier.
2. Description of the Prior Art
Recently, there has been provided a method of improving a sound quality by enlarging the dynamic range by increasing the dynamic power of an audio amplifier. Such a high-power audio amplifier uses a power supply of the high-capacity and low-impedance type corresponding to its dynamic power. However, such a high-capacity type power supply is an excessive equipment and is wasted when it is used at an ordinary listening level (output on the order of about hundreds of milliwatts), and at the same time it is used in turn at an output level which is remarkably lower than the rated output, so that there are disadvantages such that an electric power is lost and abnormal exothermic is caused and the like.
To solve such a drawback, conventionally, the efficiency is improved by use of a power supply of the voltage change-over type for an output transformerless type amplifier as shown in FIG. 1. A schematic arrangement of this power supply will be explained below. A reference numeral 10 denotes a control circuit to detect a voltage (output voltage) across a load ZL; T.sub.10 and T.sub.13 are high voltage taps; T.sub.11 and T.sub.12 low voltage taps; and SW.sub.1 and SW.sub.2 are switches which are respectively provided at the front stages of transistors Q.sub.11 and Q.sub.13 and of transistors Q.sub.12 and Q.sub.14. The transistors Q.sub.11 and Q.sub.12 are complementary. And also, the transistors Q.sub.13 and Q.sub.14 are complementary. The voltage across the load ZL is detected by the control circuit 10 and, in response to this detection signal, the two taps of the high voltage taps (T.sub.10, T.sub.13) and low voltage taps (T.sub.11, T.sub.12) that are outputted through diodes D.sub.11 to D.sub.16 from a secondary winding T.sub.2 of a transformer T (T.sub.1 indicates a primary input winding of an alternate current supply source AC) are switched by the switches SW.sub.1 and SW.sub.2 and used. With such an arrangement, the electric power is supplied to the load ZL through the transistors Q.sub.13 and Q.sub.14 when the outputs of an amplifier A are small, but in this case the voltage between the collector and emitter of the transistors Q.sub.13 and Q.sub.14 becomes low, thereby enabling the electric power loss to be reduced. On the other hand, when the outputs of the amplifier A are large, the electric power is supplied to the load ZL through the transistors Q.sub.11 and Q.sub.12, but the electric power loss itself does not increase since the amplitude of the outputs are large. Thus, the operations as described above can be performed.
However, as mentioned above, the listening level which is ordinarily used in a standard home is generally on the order of about hundreds of milliwatts on an average (for example, since the power supply change-over point is set to about 30 W/8 .OMEGA. in an amplifier having the maximum dynamic power of 100 W/8 .OMEGA., the power supply on the low voltage side is used in case of the output of 0 to 30 W). Therefore, only the circuit on the low voltage side is always made operative in the ordinary use state, while the circuit on the high voltage side is at rest. Thus, only capacitors C.sub.13 and C.sub.14 which are used in the power supply on the low voltage side are always used, but capacitors C.sub.11 and C.sub.12 which are used in the power supply on the high voltage side are hardly utilized. In addition, even in the case where the power supply on the high voltage side is used due to an instantaneous high output, the capacitors C.sub.13 and C.sub.14 for use in the power supply on the low voltage side are at rest, so that the capacitors cannot be effectively used in the power supply of the conventional type mentioned above. The individual power supplies need the capacitors each having a considerable capacitance and this results in a cause of an increase in costs.